1. Field of the Invention
This invention relates to a screen printing method used in forming electrically conductive films included in electronic parts by printing, and it also relates to a method of producing printing plates and an apparatus embodying these methods.
2. Description of the Background Art
FIG. 14 shows an example of a conventional master plate. This master plate 110 is in the form of a transparent plate 111 having desired opaque master plate patterns 112 (shown hatched) formed on its surface.
Heretofore, using such master plate 110, the plate making of screen printing plates has been performed in the following manner.
As shown in FIG. 15, said master plate 110 is fixed on a fixing frame 120 by a fixing member 121, and the assembly is attached to a predetermined position on the fixing block 131 of an exposure system 130.
On the other hand, a screen printing plate 140 is prepared which comprises a screen mesh 142 spread on a plate frame 141 and a photosensitive film 143 applied to said screen mesh 142, said screen printing plate 140 being attached to a predetermined position of the movable stage 32 of the exposure system 130.
The movable stage 132 is lifted until the screen printing plate 140 is positioned close to or in contact with the master plate 110, whereupon a light source 133 is turned on for exposure.
Thereafter, the screen printing plate 140 is removed and developed, whereby the portions of the photosensitive film 143 which have not been irradiated with light are removed; thus, there is provided a screen printing plate 140 having printing patterns 144, as shown in FIG. 16.
The screen printing plate 140 obtained in this manner is used to perform a screen printing operation conventionally in the following manner.
As shown in FIG. 17, the screen printing plate 140 resulting from said plate making operation is attached to a predetermined position on the fixing block 151 of a printing machine 150 by using a stopper 152, and a thing to be printed (hereinafter referred to as the workpiece) 160 is positioned at a predetermined position on the lifting type movable stage 154 of said printing machine 150. The movable stage 154 is lifted until the workpiece 160 is opposed to the screen printing plate 140 with a predetermined clearance defined therebetween, whereupon a squeegee 156 is moved under pressure to force a paste 157 out through the printing patterns 144 of the screen printing plate 140 so as to print the surface of the workpiece 160.
In the prior art, however, there has been a problem that the scattering (variation) of the position of the printing patterns 144 on the screen printing plate 140 is as high as 1-2 mm.
The reason is that, heretofore, during exposure of the screen printing plate 140, the positioning of the master plate 110 relative to the fixing frame 120 or the positioning of the fixing frame 120 and screen printing plate 140 relative to the exposure system 130 has not been strictly effected; thus, the relative position between the master plate 110 and the screen printing plate 140 tends to scatter.
Further, in the prior art, there has been a problem that much time and labor is taken in attaching the screen printing plate 140 to the printing machine 150 and aligning it with workpiece 160.
The reason is that in addition to the fact that the scattering of the position of the printing pattern 144 on the screen printing plate 140 is very high as described above, the mechanical positioning of the screen printing plate 140 relative to the printing machine 150 is not strictly made, so that it has heretofore been necessary to perform trial printing many times.
More particularly, after trial printing of the workpiece, patterns on its surface are observed to find the necessary correction distance and the position of the screen printing plate 140 is shifted by an amount corresponding to said correction distance (in some cases, the position of the workpiece 160 will be shifted). More specifically, the position of the stopper 152 (one of which is provided in each of the X- and Y-directions) is corrected by a micrometer 153.
However, even if such correction is made, it is very difficult to increase the printing accuracy on the first trial; usually, such a trial printing and correction process has to be repeated many times, which has heretofore taken dozens of minutes or even more time.
Even with such a process, however, the positional accuracy is limited to about .+-.100 .mu.m.